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The Fight Against Disease
Natural Defenses
The skin and mucous membranes form the body's first line of defense against
disease. Most microscopic pathogens, or microbes, cannot pass through unbroken
skin, although they can easily enter through cuts and other wounds. Mucous
membranes protect internal organs that are connected with the outside of the body.
These membranes, which line the respiratory, digestive, urinary, and reproductive
tracts, secrete a sticky fluid called mucus, which traps microbes. The mucus may
then be expelled from the body, perhaps in a cough or sneeze or in feces. If the
mucus is swallowed, digestive juices kill the microbes.
Small hairlike projections on the lining of the nose, throat, and bronchial tubes work
in conjunction with mucus to trap and remove foreign substances. In the ears, tiny
hairs plus a sticky wax defend against the entry of germs. Tears secreted by the
lachrymal gland wash away germs and other small objects that may enter the lid
area of the eye. Tears also contain a protein that kills certain germs.
If a pathogen breaches the body’s outer barriers, the defenses of the immune
system spring into action. Some of these defenses are effective against a variety of
invaders, while others are tailor-made to fight a specific organism. White blood cells
called phagocytes constantly travel through the bloodstream on the lookout for
foreign objects. If they come upon a microorganism, they surround, engulf, and
digest it.
If the infection persists and there are too many organisms for the phagocytes to fight
by themselves, the immune system produces proteins called antibodies. Each
antibody is designed to combat a particular antigen, or foreign protein. Two types of
white blood cells are involved in this process. B cells release the antibody, which
attaches to the outer covering of the antigen, marking it for destruction. T cells
attack the tagged antigen and also stimulate B cells into action. Once the body has
produced antibodies to a specific microorganism, it generally is immune to future
invasions by that organism. That is why people who have had chicken pox or
measles as a child will not get the disease again as an adult. The reason people
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